The study will examine the metabolism of exogenous fatty acids by Macaca fascicularis. Trans fatty acids which cannot be synthesized by eukaryotes will be used as the exogenous fatty acid marker. These fatty acids are geometrical isomers of cis fatty acids which can be synthesized by eukaryotes. The trans fatty acids are catabolized and removed from the body shortly after the fatty acids are removed from the diet. Twelve male M. fascicularis monkeys were used in the study. Six were fed the control diet, and the others received the experimental diet. The RBC membrane of monkeys fed the control diet contained .5% trans octadecanoate isomers of the fatty acids. Examination of the fecal fatty acids from these monkeys showed that biohydrogenation does occur in their gut producing trans fatty acids which appear to be incorporated into the RBC membrane. Within 6 weeks after initiating the study the RBC membranes of monkeys fed the experimental diet contained 10.8% trans octadecanoate isomers of the total fatty acids. Corresponding with the increase in trans fatty acids there was an increase in the membrane linoleate to arachidonate ratio. This indicated that trans fatty acids interfere with arachidonate synthesis. There was also a displacement of saturated fatty acids and cis octadecanoate isomers. The double bond index was calculated and indicated that the observed perturbations in the fatty acid content of the RBC membranes from monkeys fed the experimental diet resulted in decreased membrane fluidity.